Conventionally, in a digital recording disk device, digital recording VTR etc., data are, without recording the data as they are, recorded after converting into a recording code. A 1,7 code and a 2,7 code are typical recording codes.
In the 1,7 code, after a data bit of two bits is converted into a channel bit of three bits or a data bit of four bits is converted into a channel bit of six bits, it is recorded by using a NRZI rule. The NRZI rule is a recording rule that inversion is conducted by "1" and non-inversion is conducted by "0". A big characteristic of the 1,7 code is that there exists "0" of one or more and 7 or less between "1" and "1" after converting, i.e., a minimum inversion interval is 2.
In the 2,7 code, after a data bit of two bits is converted into a channel bit of four bits or a data bit of four bits is converted into a channel bit of six bits, it is recorded by using a NRZI rule. A big characteristic of the 2,7 code is that there exists "0" of two or more and 7 or less between "1" and "1" after converting, i.e., a minimum inversion interval is 3.
On the other hand, a method has been suggested where, to a reproduction signal, detection of data is conducted by using a combination of a partial response equalization system and a Viterbi decoding. For example, this method is disclosed in Japanese patent application laid-open No. 4-298865 (1992) (application No. 3-64201 (1991) titled "Reproduction Data Detecting System".
In this reproduction data detecting system, a code with a minimum inversion interval of 2, such as a 1,7 code is recorded. In reproducing, a bit error correction process is conducted by converting a reproduction signal into a ternary data by PR(1,1) equalization and then decoding it by four-state Viterbi decoding to reduce an error rate.
However, in the conventional method of detecting data by the combination of partial response equalization and Viterbi decoding, there is a problem that a reproduction-adaptive process cannot be performed, whereas a bit error correction process is conducted to reduce an error rate.
Namely, in PR(1,1) equalization, a fixed equalization is always carried out even when a change in reproduction signal due to a variation in recording current when recording, a variation in medium characteristics caused by a recording device, a variation in recording and reproducing characteristics caused by temperature or the like occurs. Therefore, equalization characteristics cannot follow the change in reproducing signal.